It has been common practice for some time in the foundry industry to fabricate molds and cores, for use in casting metal parts, from commercial metal casting "plaster" which is a blend commonly comprising at least 50% gypsum plaster with the balance being primarily fibrous talc and some silica sand. Such molds and cores are conventionally used with a variety of metals which melt at temperatures substantially above the boiling point of water but below the melting point of gypsum, 2640.degree. F., typical examples being aluminum and its alloys with melting points in the range of 1050.degree.-1200.degree. F., zim base alloys which melt around 900.degree. F. and unleaded bronzes with a melting range of 1800.degree.-1900.degree. F.
In fabricating molds and cores for such uses, the commercial plaster is mixed with a large amount of water, for example an equal or greater amount of water, to produce a highly fluid suspension which is capable of completely filling even relatively complex patterns in the master mold or pattern. Then this large amount of water must be substantially completely eliminated, because any water which remains in the plaster can spoil a casting made therefrom, when it turns to steam upon contact with the molten metal at the elevated temperatures noted above, either by producing surface defects or by virtually exploding portions of the mold.
Drying of a plaster casting component by conventional methods is tedious and of unpredictable reliability in results, particularly if the component is complex or of substantial mass. One reason for these difficulties is that the gypsum component of the plaster normally retains a significant amount of water of crystalization, which cannot be eliminated without heating the entire component to a temperature greater than its calcining temperature of 270.degree. F. This is a very time-consuming operation with a conventional baking furnace, which can easily require as much as 30 hours at 300.degree. F., and even then, the probabilities are that a substantial proportion of a given plurality of components will crack or craze sufficiently to be unusable.
Attempts have been made to dry plaster casting components by exposure to microwave radiation in a microwave oven, on the premise that the known absorption capabilities of water for microwave radiation should make microwave heating an effective drying procedure for the plaster. Strangely, however, these attempts have not been successful, even when the mold or core is heated far beyond the normal 300.degree. temperature obtained in a conventional oven, for example even as high as 600.degree. F. While a mold or core dried in this manner appears to be completely dry, when it is then used for casting, sufficient additional water is given off by the plaster to spoil the majority of the castings. Additionally, heating to such high temperature ranges will usually cause cracks or crazing in a significant portion of the components which make them useless for casting purposes.
My above cross-referenced application discloses that casting components formed of commercial metal casting plaster can be dried very satisfactorily, very much more quickly than by conventional methods, and with minimal damage to the structure of the component itself and to its surfaces, if the wet-molded component is subjected to a two-stage microwave radiation treatment with an intermediate cooling step. More specifically, it appears that the water is effectively eliminated, i.e. the casting component is completely calcined, without loss of its strength or surface characteristics, when the microwave treatment is carried out only until the internal temperature of the component slightly exceeds 300.degree. F., followed by cooling to a temperature of not more than 200.degree. F., and then by a further microwave treatment which raises the temperature throughout the component to about 300.degree. F.
The success of this procedure apparently derives from the fact that during the first microwave treatment, the free water throughout the casting component is driven off, but while the water of crystalization in the central zones of the component is caused to migrate to the surface zones, it is not driven off because the surface of the component is sufficiently cooled by evaporation, of the free water, and also by radiation of heat to the normally cold walls of a microwave oven, to prevent the surface temperature from reaching the calcining range except after such prolonged treatment and resultant high internal temperatures as will "dead burn" or destroy the strength of the plaster of the central zones of the component. The second microwave treatment after cooling causes the water to be driven off from the surface zones of the component before the surface has been cooled by evaporation, and also before the central zones of the component can be reheated to the point of damage.